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Studying Fluid Mechanics, I started to notice that almost every textbook/website uses a specific point to make calculations about the pressure in a liquid at a given depth (hydrostatic pressure): the geometric center (as shown in the images below), when presenting pressure gauges/manometers/piezometers.

Note: This happens regardless of the field to which the book is directed (I looked in textbooks of Fluid Mechanics for Civil, for Electrical, for Mechanical...).

Pressure gauges/Manometers/Piezometers         Sources: Introduction to Fluid Mechanics - Nakayama & Boucher/Mecânica dos Fluidos - Noções e Aplicações - Sylvio R. Bistafa/Chegg


One of the textbooks I looked at even draws attention to this fact, but it doesn't explain the reason for the choice:

Note the origin of the measurement of h, in the center of the tube

                                
                                                          Source: Mecânica dos Fluidos - Franco Brunetti

A similar behavior can be identified when textbooks present liquids in motion: they use the centerline of the pipe to make calculations/measurements. Here's an example:

                                    
                                                     Source: Fluid Mechanics for Civil Engineers - N.B. Webber


So why is the choice of geometric center/centerline of the pipe so common when measuring/calculating pressure? Some hypotheses:

  • Maybe all the textbooks/websites are unconsciously copying each other?
  • Maybe is this some kind of "convention"?

Note to the off-topic warning: "Questions about the physical reasoning and analysis that lead to design decisions are on topic". That's the core of my question: what is the physical reason for choosing the geometric center in fluid mechanics books. In other words, what is the physical reasoning that leads to the design decision commonly adopted by almost every textbook of choosing the geometric center/centerline of the pipe when doing pressure-related calculations/measurements.

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  • $\begingroup$ In my judgment, the center is just a convenient datum to use. For flow in a horizontal pipe, there pressure gradient axially is determined by the flow, but the pressure gradient vertically is determined hydrostatically (and is the same at all axial locations). So, it really doesn't matter where the vertical datum is taken. $\endgroup$ – Chet Miller Nov 2 '18 at 2:29
  • $\begingroup$ @Aaron Stevens;@ZeroTheHero;@Chair;@Cosmas Zachos;@ahemmetter "Questions about the physical reasoning and analysis that lead to design decisions are on topic". That's the core of my question: what is the physical reason for choosing the geometric center in fluid mechanics books. In other words, what is the physical reasoning that leads to the design decision commonly adopted by almost every textbook of choosing the geometric center/centerline of the pipe when doing pressure-related calculations/measurements. $\endgroup$ – Vinicius ACP Nov 20 '18 at 12:15
  • $\begingroup$ I'm sorry, but I didn't understand why my question is off-topic. $\endgroup$ – Vinicius ACP Nov 20 '18 at 12:20
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    $\begingroup$ This clearly isn't an engineering question, but it looks like one at first glance. Anyway it has 3 reopen votes already so it should be reopened fairly shortly. $\endgroup$ – Chris Nov 20 '18 at 13:14
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I'll divide my answer in two cases. First, I'll talk about liquids in motion (assuming incompressible flow). Then, I'll talk about liquids at rest.


Liquid Flow:

Reading the comments of this YouTube video about piezometers made by Donald Elger, I found the answer for this case:

Why is it [the pressure measurement with piezometer] taken from the middle of the pipe?

Elger's answer: The pressure variation across a section of a pipe is hydrostatic; thus, the pressure will vary linearly with radius and the pressure at the center of the pipe is the average pressure. If you use this value of pressure in your calculations, this will be give you the most accurate results. Thus, engineers nearly always apply or measure the pressure at the center of the pipe.

The question that came to me as soon as I read this was: "Why using average pressure in calculations gives the most accurate results?".

(Note: I recommend reading my answer to this question before proceeding)

Briefly, in general, the average pressure gives the most accurate results if used in calculations because there are many applications/cases in which the locations with $P=P_{average}$ are the best places for experimental data collection.

In the case of a pipe, this location is its centerline. So, I believe that this is why textbooks generally choose this location in case of liquids in motion: the centerline is associated with $P_{average}$ that, in its turn, is associated with the best places for experimental data collection for many applications.


Liquids at rest:

For this case, firstly I would like to quote part of the answer written by David White to my question "Where is the right place to put the pressure gauge to measure the pressure of a tank?":

The location depends on why you are measuring the pressure. There will be a process reason for the pressure measurement, and that will determine the location of the pressure measuring device.

When textbooks present pressure gauges/manometers/piezometers for the first time, the presentation is usually "application neutral" (i.e., there's no process reason), the diagrams/sketches/figures are only to illustrate the concepts/formulas. Therefore there are no best points as in the liquid flow case, for two reasons:

  • There is no process reason that determines the location of measurement;
  • Since the liquid is at rest, there are no points that lead to most accurate results, they all provide the same accuracy.

But the authors need to choose a point to do the pressure-related calculations...

After everything I've researched, my hypothesis is that the "point choice" of hydrostatics was imported from hydrodynamics. So, instead of choosing a random point to pressure-related calculations, they choose one that at least has importance/meaning for other areas of Fluid Mechanics.

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